1. Technical Field
This invention relates to the field of clutches for a compressor in an air conditioning system. More particularly, the invention pertains to such a clutch having soft start characteristics.
2. Description of the Prior Art
A compressor for an air conditioning system of an automobile typically includes a clutch to enable disengagement of the compressor during periods in which no air conditioning is desired. During demand situations, the clutch is engaged so that the compressor compresses the refrigerant in a known manner. Prior art clutches typically are of an electromagnetic friction clutch construction. A compressor typically is driven by a V-groove pulley assembly supported by a bearing pressed onto the body of the compressor. The pulley is driven by the automotive accessory drive belt.
Prior art clutches typically include an armature affixed to the splined shaft of the compressor via a bolt. A stationary wire coil assembly is press-fit to the compressor housing. The armature is normally separated by a small air gap from the pulley face by a spring mechanism, so that the clutch is normally disengaged. Automotive voltage is applied to the coil to engage the clutch, normally having a nominal value of 14.4 volts. Upon application of this voltage, the current in the coil increases from zero on a time scale related to the inductive time constant of the clutch, typically 150 milliseconds. The current induces magnetic flux to flow in the pulley, across the air gap, and into the armature. When the magnetic flux density reaches a critical level, the attractive force between the armature and the pulley becomes large enough to overcome the spring force holding the armature away from the pulley. The armature is then rapidly drawn into contact with the pulley, suddenly initiating torque transfer to the compressor and causing the compressor shaft to begin to rotate. When the shaft speed matches the pulley speed, the torque then reaches a steady-state level that is a function of pulley speed, cooling demand, and other vehicle operational characteristics.
When such a prior art clutch is engaged, undesirable effects can occur, including stumble, surge, and noise. Stumble is a longitudinal vehicle vibration induced by the sudden change in engine torque demand which occurs upon compressor engagement. Surge is a lurch that occurs when the clutch is disengaged. Noise is generated as the armature of the electromagnetic friction clutch is rapidly driven into contact with the clutch pulley during engagement. These effects of rapid compressor engagement are objectionable to the vehicle occupants and may contribute to premature failure of compressor components. Previous efforts to overcome these concerns include using passive mechanical means, such as mating slots or other structures provided in the pulley and armature, which are said to reduce the rate of increase of magnetic force. An example of this is illustrated in U.S. Pat. No. 4,749,073 to Olden.
Another attempt to reduce these concerns includes the insertion of an elastomeric coupling between the armature and compressor shaft to damp transients encountered during engagement and operation, as shown in U.S. Pat. No. 5,219,273 to Chang. Other attempts to reduce these concerns include electronic controls of the clutch current in an attempt to produce soft-start coupling, as described in U.S. Pat. No. 4,509,091 to Booth and U.S. Pat. No. 4,567,975 to Roll. These patents disclose a method of generating a time-varying clutch current. These methods draw the armature initially to the pulley, but allow the pulley to slip. The clutch current is increased smoothly to gradually increase the level of torque transfer and decrease the slip until a state of complete engagement is reached. These methods permit slip, which causes the electromagnetic friction clutch surfaces to become burnished over time, reducing the ability of the clutch to transfer torque.
It would therefore be desirable to provide a clutch which softens the engagement of an air conditioning compressor, but which does not cause excessive wear of the clutch friction surfaces.